Rügen is the biggest German island on the Baltic Sea.
It was a very famous DDR holyday place and aside from the amazing wildlife (wonderful cliffs and wild coast), it's full of small jewels spread around it. Like Prora (the colossal Nazi-planned tourist structure), the white sea houses, the architectures by Ulrich Müther (the Musikpavillon, the Life Guard Tower).
And more: the biggest DDR Harbour Mukran, the Arkona-Bunker, a couple of abandoned russian military bases and the Jagdschloss Granitz.
More infos. WIKI